The present invention generally relates to adsorbents for carbon dioxide and water and, more specifically, to an improved adsorbent for the simultaneous removal of carbon dioxide and water from air and that can be regenerated with heat and/or vacuum.
Carbon dioxide is produced from biological processes, combustion of fossil fuels, and from other industrial processes. For the maintenance of human lifexe2x80x94especially in space exploration, underwater activities, and underground mining activitiesxe2x80x94the control of the concentration of carbon dioxide is critically important. In addition, carbon dioxide has been identified as one of the global warming gases. As such, the control and utilization of carbon dioxide is an important technology requirement for protecting the global environment.
Conventional adsorbents for the adsorption of carbon dioxide are zeolite materials such as 13X and 5A. For the use of these adsorbents, a 4-bed unit has been designed. Two beds are used for the removal of humidity in the air before the dry air stream is contacted with the zeolite adsorbent for carbon dioxide removal. These adsorbent materials are inorganic oxides that also adsorb water; however, the adsorption of water simultaneously diminishes the capability of the zeolite for carbon dioxide adsorption. In addition, the binding energy of carbon dioxide on the zeolite is high. Consequently, a high temperature of about 523xc2x0 K is needed to remove the carbon dioxide from the adsorbent.
In an effort to overcome disadvantages presented by a zeolite adsorbent, carbon-based materials have been used. For example, in U.S. Pat. No. 4,820,681 which is assigned to the assignee of the present invention, a carbon molecular sieve was prepared by polymerizing a cross-linking agent and a precursor monomer to produce a cross-linked polymer. The cross-linked polymer was then shaped into a desired configuration without the need for a binder. The shaped polymer was then carbonized.
In a fashion related to the above patent, U.S. Pat. No. 4,810,266 which is also assigned to the assignee of the present invention discloses a carbon molecular sieve. The sieve is similarly prepared by polymerizing a cross-linking agent and precursor monomer. The cross-linked polymer that is produced was also shaped into a desired configuration and carbonized. The pores of the material were then enlarged by steam treatment. And the material was given an amine functionality that improved capacity upon regeneration of the material by heating.
While the above art has provided advantages, it has not adequately addressed the importance of adsorbent characteristics such as pore size, micropore volume, pore size distribution, surface hydrophilicity, and surface functionality. These characteristics are important because carbon dioxide adsorption on solid adsorbents is based on the interaction forces between the gaseous molecule and the surfaces on micropores in the adsorbent.
As can be seen, there is a need for an improved adsorbent for carbon dioxide and water from air that overcomes disadvantages of the prior art.
In one aspect of the present invention, a carbon dioxide and water adsorbent comprises a carbonized, activated, and functionalized carbon material having first functional sites that adsorb carbon dioxide and second functional sites that adsorb water, whereby the adsorbent is characterized by a total pore volume of between about 1.5 to 2.5 cm3/g, a median pore diameter between about 0.5 to 1.2 nm, and a BET surface area between about 2000 to 2500 m2/g.
In another aspect of the present invention, a method of making a carbon dioxide and water adsorbent comprises polymerizing a monomer to produce a precursor material; carbonizing the precursor material to produce a carbonized precursor material; activating the carbonized precursor material to produce an activated precursor material; functionalizing the activated carbon precursor material to produce a functionalized carbon material having functional sites that adsorb carbon dioxide independently of adsorbing water; whereby the adsorbent is characterized by a carbon dioxide adsorption capacity between about 4.0 to 6.0 wt./wt. %, as well as a water adsorption capacity between about 10 to 30 wt./wt. %, at a vapor pressure of CO2 at about 3.8 Torr and a water vapor pressure of about 25 Torr, and a temperature of about 25xc2x0 C.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.